1. Field of the Invention
This invention relates to polycarbonates, and more particularly to polycarbonates having improved melt stability and resistance to hydrolysis.
2. Description of the Prior Art
Polycarbonates derived from reactions involving organic dihydroxy compounds and carbonic acid derivatives have found extensive commercial application because of their excellent mechanical and physical properties. These thermoplastic polymers are particularly suited for the manufacture of molded products where impact strength, rigidity, toughness, thermal and dimensional stability as well as excellent electrical properties are required.
However, these polymers exhibit a brief though definite burning time when contacted with an open flame and do not meet a specified requirement for flammability resistance in applications where high temperatures and/or exposure to fire may be encountered.
In order to improve the flammability resistance of the polycarbonate various approaches have been pursued including the incorporation of halogenated phenolic diols into the polycarbonate polymer chain. Typically copolymers of tetrahalobisphenol A and bisphenol A are used to impart improved flammability resistance to the polycarbonate. However, unless the quantity of the halogenated bisphenol in the copolymer is very minor, the copolymers have such high fabrication temperatures that the halogen atoms tend to be liberated at processing temperatures tending to degrade the polymer and reduce its properties. Further after the molded polycarbonate is formed, the halogenated phenolic diol tends to contribute to hydrolytic instability of the article causing a degradation in mechanical and physical properties. On the other hand, when only a small amount of halogenated phenolic diol is incorporated into the copolymer the necessary flame retardancy is not provided to the polycarbonate.
Mixtures of homopolymeric non-halogenated polycarbonates and homopolymeric tetrachlorobisphenol-A polycarbonate as described in U.S. Pat. No. 3,038,874 and mixtures of homopolymeric non-halogenated polycarbonates with copolymers of tetrabromobisphenol-A and other phenols, glycols, carboxylic acids and the like as disclosed in U.S. Pat. No. 3,334,514 and various other mixtures of homopolymeric halogenated bisphenol-A polycarbonates have been proposed in order to cure the foregoing difficulties.
Although brominated polycarbonates do not require fabrication temperatures as high as those which must be used when chlorine-containing polymers and copolymers are used, nevertheless, the less tenacious bromine atom much more readily splits off under polycarbonate processing conditions and severely degrades the polymer rendering it commercially useless even when mixtures of homopolymeric and copolymeric brominated polycarbonates are used in which the concentration of the halogen containing copolymers in the admixture is very low, i.e. even before the halogenated polymer is present at a concentration sufficient to permit to act as an effective flame retardant.
In order to remedy the deficiencies of the halogenated polycarbonate polymers many additives have been incorporated into the polycarbonate; phosphite stabilizers such as those disclosed in U.S. Pat Nos. 3,509,091, 3,205,269, 3,305,520, 3,342,767, 3,398,115, 2,867,594 and Canadian Pat. No. 646,424 representatively, have shown utility in stabilizing halogenated polycarbonate copolymers. Further, phosphites in combination with barium carbonate, strontium carbonate and/or calcium carbonate have substantially improved the stability of halogenated polycarbonates as is taught by U.S. Pat. No. 3,733,296. However, even with the foregoing recited stabilizers, the halogenated polycarbonates exhibit degradation due to susceptability to hydrolysis.
Thus in accordance with the present invention halogenated polycarbonates are provided with increased thermal and hydrolytic stability.